Abstract
The electroless-nickel (EN) has been deposited for the application of pad in the microelectronic industry. The wettability and interfacial reaction between solder and EN on the underlying substrate is thus a critical issue. In this study, EN deposited with various pH values was employed on the Al2O3 and Cu/ Al2O3 substrate, and both the wettability and interfacial reaction between EN and eutectic SnPb solder were investigated. On wettability, the variations of phosphorous content in EN did not significantly influence the contact angle of Sn-37Pb on EN metallizations. The contact angles between Sn-37Pb and EN with phosphorous contents from 6wt% to 12wt% were in a range from 30 to 55 degree. To probe the effect of thermal treatment of EN on the contact angles, the deposited EN were annealed at 350℃ for 1 to 3 hours. The EN surfaces without RMA cleaning displayed the presence of NiO, and the increased oxygen contents were evident by ESCA spectroscopy. The peak intensity of precipitated phase of Ni3P from the EN was enhanced with increasing annealing time. The contact angles between the annealed EN and Sn- 37Pb solder were approaching 180 degree without employing RMA flux. However, after using RMA flux, the contact angles were reduced to between 120 to 130 degree. This indicated that preventing EN from oxidation and precipitation was beneficial to wettability. Joints of Sn-37Pb/Au/EN/Cu/Al2O3 and Sn-37Pb/Au/Ni/Cu/Al2O3 joints were fabricated. EN deposited with the lower phosphorous contents and higher plating rate on the Cu-metallized substrate were achieved in the bath solution of the higher pH value. Both the deposited EN and the electroplating Ni exhibited columnar structure, while the electroplating Au displayed laminar. During annealing at 240℃, the Ni-P-Cu-Sn-Pb solid solution was formed between the EN-based (Ni-12.12wt%P and Ni-5.52wt%P) multi-metallizations and the Sn- 37Pb solder. The intermetallic compound Ni3Sn4 was randomly formed at the interface between the solder and the electroplating Ni. During annealing at 200 ℃, the (Ni,Cu)3Sn4 and Ni3Sn4 IMCs were present at the interfaces between the solder and the EN (Ni-12.12wt%P and Ni-5.52wt%P) metallizations. However, the ( Ni,Cu)3Sn4 IMC transformed into the Ni-P-Cu-Sn-Pb solid solution after migrate into the solder bulk took place, and the Ni3Sn4 IMC stopped growing as soon as the thickness of IMC reached 2~3 □m. In addition, either Ni-5.52wt%P or electroplating Ni metallization became scalloped with increasing annealing time. In the case of Sn-Pb/Au/Ni/Cu joint, the thickness of Ni3Sn4 IMC formed at 200℃ was larger than that at 240℃. The Sn-37Pb/Au/Ni-12.12wt%P/Cu joint formed the Ni3Sn4 IMC and solid solutions during aging at 170℃. There existed cracks and stripping in a flake during aging at 130℃. The Ni-5.52wt%P metallization in a Sn-37Pb/Au/Ni-5.52wt%P/Cu joint dissolved in the solder and formed the Ni-P-Cu-Sn solid solution during aging at 170℃ for 16 days. After aging for 25 days, the Ni3Sn4 was formed in the solid solution. During aging at 130℃, there was scarcely Ni-Sn IMC formed between the solder and the Au/Ni-5.52wt%P/Cu multi-metallization. There was a phenomenon of the EN scalloping with the increase of aging time. In other respects, Pb and P did not coexist with the Ni-Sn IMC, and there was a presence of phosphorus segregated and aggregated at the interface of the Ni-Sn IMC and EN. The electroplating Ni forms NiSn8.5 IMC with the solder during aging at 130℃ and 170℃. There was not distinct formation of Au-Sn IMC in joints of Sn-37Pb/Au/EN/Cu and Sn-37Pb/Au/EN/Cu. After aging test of pull-off assemblies, the adhesion strength between the solder and either Ni-12.12wt%P/ Cu or Ni-6.63wt%P/Cu metallizations evidently was degraded. XRD analysis revealed that either (Ni,Cu)3Sn4 or Ni3Sn4 IMC was present at the fractured surface between EN and Sn-37Pb solder.